For me, it may look like I work longer hours than before, but I find my work hours less concentrated. I usually try to log on by 9am and will find myself finishing up between 6pm and 8pm depending on the day. But I often step away a number of times throughout the day for various reasons (no kids, but I assume those with kids have even more reasons for taking a few minutes here and there away from work). It also allows me to do something else during low-energy periods rather than trying to force myself to work (I've even rarely tried naps, although it never works well for me).

The effect is that it leads to a "longer" work day, but less concentrated than it would have been in the office. And similar productivity, IMO. I wonder if some of your co-workers you describe may do the same. In some ways, I feel like it's better work-life balance for some people as it's "ok" (in some work cultures) for WFH employees to take the time as they need it. And others are equally fine ensuring their work time is as concentrated as possible because they have other things they would rather do in evenings, etc.